Free Surface Relaxations of Star-Shaped Polymer Films

Abstract

The surface relaxation dynamics of supported star-shaped polymer thin films are shown to be slower than the bulk, persisting up to temperatures at least 50 degrees above the bulk glass transition temperature Tgbulk. This behavior, exhibited by star-shaped polystyrenes (SPSs) with functionality f = 8-arms and molecular weights per arm Marm < Me (Me is the entanglement molecular weight), is shown by molecular dynamics simulations to be associated with a preferential localization of these macromolecules at the free surface. This new phenomenon is in notable contrast to that of linear chain polymer thin film systems where the surface relaxations are enhanced in relation to the bulk; this enhancement persists only for a limited temperature range above the bulk Tgbulk. Evidence of the slow surface dynamics, compared to the bulk, for temperatures well above Tg and at length and time scales not associated with the glass transition has not previously been reported for polymers.

Unexpected Molecular Weight Dependence to the Physical Aging of Thin Polystyrene Films Present at Ultra‐High Molecular Weights

“…Based on our experimental investigations, we attribute the ~45% faster physical aging response of ultra‐high MWs ≥ 6500 kg mol −1 thin PS films, relative to equivalent films of (merely) high MWs ≤ 3500 kg mol −1 , to a diminished gradient in dynamics near the free surface of these films. These results contribute to the growing reports in the literature that chain connectivity and entropy are additional factors influencing interfacial effects and dynamical gradients in thin films beyond the well‐characterized enthalpic interactions.…”

“…However, several recent studies have reported differences in confinement behavior associated with chain length, stiffness, entanglements, and chain architecture that are unexpected given the primarily enthalpic understanding developed over the years. This suggests that entropic effects may also play a modifying role in understanding confinement effects.…”

“…This suggests that entropic effects may also play a modifying role in understanding confinement effects. For example, changes in dynamics due to tethering and adsorption of chains to substrates have been reported, as well as variations for different chain architectures such as stars and cyclic chains that otherwise have identical chemical composition, suggesting that the presence of a boundary or interface alters the available entropic conformations and influences the local dynamics, altering the nature of the dynamical gradient. Recent studies have also reported unexpected MW dependences and chain stiffness effects to the glass transition temperature ( T g ) and surface dynamics that contradict the general understanding that had historically been formed.…”

Substrate Roughness Speeds Up Segmental Dynamics of Thin Polymer Films

“…Structural dynamics, the molecular mechanism associated with the vitrification of liquids, is altered by the presence of interfaces [1,2]. Because of the smaller number of neighbors (lower density ρ), molecules at the free surface with air are expected to experience faster dynamics [3][4][5][6][7][8].…”

Solution filtering affects the glassy dynamics of spincoated thin films of poly(4-chlorostyrene)

“…In the case of free surfaces, that is, an interface with air, vacuum or inert gases, molecules move faster [11][12][13][14]. This enhancement in relaxation rate is probably due to the larger space available to molecules, as density decreases while approaching the free surface [15,16]. A more intriguing scenario is observed in proximity of hard walls, where the dynamics can both accelerate or slowdown, depending on the adsorbed amount [17].…”